The present invention relates to solid-state relay voltage switching circuits.
Solid-state relays for switching a.c. or d.c. voltages are often used in applications such as traffic control equipment, motor controls, and the like, and employ thyristor devices to switch the voltage between a pair of terminals. Such relays offer many advantages over their mechanical counterparts. For example, with the solid-state relay there are no moving parts and no contact bounce, the relay is capable of fast operation, is shock and vibration resistant, and does not emit audible noise when operated.
Such relays often use a thyristor, e.g., a silicon controlled rectifier (SCR), connected with its anode-cathode circuit between the terminals, as the voltage conducting device. To operate the relay, a switch such as a reed switch is operated to connect the gate of the SCR to the anode thereof through a resistor to render the SCR conductive and thereby establish a circuit path between the terminals. When the switch is turned "off," the connection between the gate and the anode is removed, and the SCR, if then conducting, becomes nonconductive when the switched voltage passes through its zero-current point. Unfortunately, a disadvanatge of such relays is the ability of a rapidly increasing voltage at the anode of an SCR to switch on the SCR, when it is otherwise in its off state, which can lead to spurious operation of the relay. Generally, the switched voltage is an a.c. line voltage, and if it were a clean sine wave this would present no problem. However, line voltages normally encountered are not clean waves, and superimposed thereon are transient voltage spikes from switches, motors, solenoids, electromechanical relays, lightning, etc. Where the voltage transients have a sufficiently large change of voltage amplitude with respect to time, or dv/dt, they can turn on an otherwise off SCR relay.
Conventionally, attempts to prevent spurious operation of SCR relays include connecting RC networks across the anode-cathode circuit of the SCR to integrate, or reduce the dv/dt, of voltage transients applied thereacross. One disadvantage of this technique is that the relays then have inherent off state leakages through the RC networks, and the lower the required sensitivity of the relays to voltage transients, the lower the required impedance value of the RC networks, and the greater the leakage currents.
Another known technique, which essentially eliminates the leakage current, takes advantage of the characteristic of an SCR that the less positive the voltage at its gate is with respect to the voltage at its cathode, the greater the dv/dt across its anode and cathode the SCR can withstand without being switched on, or the higher its dv/dt withstand capability. With this in mind, SCRs of the so-called "shorted emitter" configuration, having a partial gate to cathode short built into them, have been used in relays. Alternatively, a resistor may be connected between the gate and the cathode of an SCR to reduce the voltage at its gate toward the voltage at its cathode, and thereby increase the dv/dt withstand capability of the SCR.
Unfortunately, the above technique suffers a serious draw-back consequent upon another inherent characteristic of an SCR; namely, the less positive the voltage at its gate is with respect to that at its cathode, the greater the gate current required to render it conductive. To decrease the gate current required to turn on the SCR, the gate should not be connected to the cathode. This, of course, undesirably decreases the dv/dt withstand capability of the SCR. To increase the dv/dt withstand capability, the voltage at the gate is ideally brought as close as possible to the voltage at the cathode. As noted, this undesirably increases the gate turn on current. Thus, the two desiderata are at odds with one another. In the prior art, at compromise was made, i.e., the value of the resistance between the gate and the cathode was selected to provide a "compromise voltage" at the gate, which neither maximized the dv/dt withstand capability of the SCR, nor minimized the gate turn on current thereof. Even where increased gate turn on current can be tolerated, the value of the resistance may be decreased to bring the voltage at the gate of the SCR toward the cathode voltage only to an extent that allows the SCR to be turned on, which gate voltage does not equal that at the cathode and therefore does not fully increase the dv/dt withstand capability of the SCR.